The use of radio frequency identification (RFID) systems is expanding rapidly in a wide range of application areas. RFID systems consist of a number of radio frequency tags or transponders (RFID tags) and one or more radio frequency readers or interrogators (RFID readers). The RFID tags include one or more integrated circuit (IC) chips, such as a complementary metal oxide semiconductor (CMOS) chip, and an antenna connected thereto for allowing the RFID tag to communicate with an RFID reader over an air interface by way of RF signals. In a typical RFID system, one or more RFID readers query the RFID tags for information stored on them, which can be, for example, identification numbers, user written data, or sensed data. RFID systems have thus been applied in many application areas to track, monitor, and manage items as they move between physical locations.
RFID tags can generally be categorized as either passive tags or active tags. Passive RFID tags do not have an internal power supply. Instead, the relatively small electrical current induced in the antenna of a passive RFID tag by the incoming RF signal from the RID reader provides enough power for the IC chip or chips in the tag to power up and transmit a response. Most passive RFID tags generate signals by backscattering the carrier signal sent from the RFID reader. Thus, the antenna of a passive RFID tag has to be designed to both collect power from the incoming RF signal and transmit (or reflect, e.g., backscatter) the outbound backscatter signal. Due to power limitations, the ability to provide devices such as sensors or microprocessors on passive RFID tags is limited. Passive RFID tags do, however, have the advantage of a near unlimited lifetime as they obtain their power from the RF signal sent from the RFID reader.
Active RFID tags, on the other hand, have their own internal power source, such as, without limitation, a battery, a fuel cell or what is commonly known as a super capacitor. The internal power source is used to power the IC chip or chips and discrete circuit elements, which typically include an RF receiver, an RF transmitter, and some type of controller, such as microcontroller or other processor, and any other electronics provided on the active RFID tag. As a result, active RFID tags can include relatively high power devices such as sensors, microprocessors, receivers and transmitters. Also, because of the on-board power, active RFID tags typically have longer ranges and larger memories than passive RFID tags. The internal power source, however, also means that active RFID tags typically have a lifetime that is limited by the lifetime of the power source. Thus, periodic maintenance is required.
As noted above, multiple RFID tags may be used to track, monitor, and manage multiple items/assets as they move between physical locations. In such an application, each active RFID tag is affixed to an item/asset that is located in a particular location or environment, such as in a building. The term “building” as used herein shall refer to any structure including, without limitation, a warehouse, a hospital, an office building, or even a vehicle. In current active RFID systems, the active RFID tags, when deployed in such a manner, are done so in a state where (i) an RF receiver of the tag is in an active state for receiving RF signals, and (ii) the controller is in a low power inactive (sleep) state to preserve power. When one or more of the active RFID tags are to be queried, the RFID reader sends out a wake-up signal that is received by the RF receiver of each tag. Upon receipt of the signal, the RF receiver in each tag will then send a signal to the controller of the tag that causes it to move from the inactive state to an active (wake-up) state. For example, in RFID systems implemented according to the ISO 18000 Part 7 standard, when one or more tags are to be queried, the reader will send out a 30 KHz tone lasting for a period of approximately 2.5 seconds. Upon receipt of the tone, the RF receiver in each tag will wake-up the controller in the tag. The RFID reader then sends out signals intended for particular ones of the tags. Those particular tags for which the signals are intended will then perform the requested action, and the remaining tags (i.e., those tags not currently of interest to the reader) will move back to a sleep state. In such systems, tags may also be on continuously not requiring a wake-up signal.
The multiple active RFID tag arrangement just described presents at least two power management problems. First, each active RFID tag that is deployed is required to have at least one component, i.e., an RF receiver, in an active, relatively high power consuming state at all times so that it can listen for the wake-up signal (if employed). Second, when the RFID reader needs to query one or more particular tags, all of the tags that are deployed are woken up (for example, according to the ISO 18000, Part 7 standard), i.e., their controllers are caused to move to an active, relatively high power consuming state. Only when a particular tag determines that the query in question is not intended for it will it then move back to the sleep state. As will be appreciated, these problems result in unnecessary use of power from the power source (e.g., battery) of each tag, and therefore decrease the lifetime of each tag.
United States Patent Application Publication No. 2007/0205873 (referred to herein as the “'873 publication”), owned by the assignee hereof and entitled “Methods and Apparatus for Switching a Transponder to an Active State, and Asset Management Systems Employing Same,” is incorporated herein by reference and discloses a number of transponder apparatus embodiments that overcome at least two problems associated with (1) current active RFID tags, and (2) current active RFID tag wake-up protocols. The first problem, as noted above, is that in current RFID tags, an active RF receiving element must always be awake to anticipate a wake-up signal for the balance of the tag electronics. The '873 publication discloses a passive circuit to eliminate the need for an “always on” active RF receiving element to anticipate a wake-up signal for the balance of the tag electronics. This solution allows the entire active RFID tag to have all circuit elements in a sleep (standby) state, thus drastically extending battery life or other charge storage device life and thus essentially eliminating shelf maintenance on the active RFID tag. The second problem, as also noted above, is that in current active RFID tag systems, the electronics of all of the RFID tags in a system are awakened in response to wake-up signals even if the signal is not intended for a particular tag or tags. The solution disclosed in the '873 publication provides a major energy saving circuit that eliminates the need to wake up all of the RFID tags in response to each wake-up signal. This circuit thus reduces total energy consumption of an active RFID tag system or collection of devices by allowing all non-addressed tags to remain in a sleep (standby) state, thereby reducing total system or collection energy. This second circuit can be used in conjunction with the first passive circuit mentioned above or in conjunction with any existing active RFID tag systems.
Furthermore, the numerous advances in wireless RF devices have made possible the determination of location of a transmitting or reflecting RF device in such a way as to make it possible to determine its geographic location. One such methodology is the Global Positioning System (GPS). However, GPS does not work satisfactorily inside of buildings. Other methodologies have been proposed that use the strength of the signal of the device transmission or reflection to provide measurement inputs which are then processed to produce an estimate of device position (e.g., systems that employ a single collection point and Received Signal Strength Indicator (RSSI) signals used to calculate a distance from an RF source). The problem with RSSI based systems is that environmental factors may have an adverse effect on the required distance calculations (which are usually based on the Friis equation).
The '873 publication discloses a novel system for tracking a plurality of assets that employs a plurality of transponders as described above, wherein each of the transponders is associated with a respective one of the assets and stores an identifier identifying the particular asset with which it is associated. The system includes a central computer system that maintains a plurality of records relating to the assets. When properly interrogated by an RF signal that is generated at the direction of the central computer system, each of the transponders generates and transmits a response signal including the identifier identifying the particular asset with which the transponder is associated. The system further includes a network with which the central computer system may communicate, a plurality of wireless access points in electronic communication with the network, and a plurality of interface devices. Each of the interface devices is adapted to (i) wirelessly communicate with at least one of the wireless access points, (ii) receive the response signal transmitted by a particular one or more of the transponders, and (iii) generate and transmit to the wireless access points at least one second response signal that includes each identifier that was included in each response signal received by the interface device. Each second response signal is transmitted to the central computer system through the network, which then uses the received second response signals to update one or more of the records. In particular, the assets are located within an environment such as one or more buildings, and each of the interface devices is associated with a particular location within the environment. In addition, each of the second response signals includes an identification of the interface device from which it was transmitted, and the central computer system uses the identification included in each second response signal to update in the records a location of one or more of the assets. While the system for tracking assets described in the '873 publication is highly effective, there is room for further improvement in the area of asset location measurement and tracking.